α-Iodoperfluoroalkanes and α,ω-diiodoperfluoroalkanes (herein after otherwise respectively referred to as “I-telomers” and “I2-telomers”) are useful reagents or intermediates in a variety of applications.
α-Iodoperfluoroalkanes having general formula F(CF2)nI, wherein n is typically an integer higher than 3, can be used, for example, as intermediates for the manufacture of surfactants, pesticides, electronic materials or pharmaceuticals.
α,ω-Diiodoperfluoroalkanes having formula I(C2F4)nI (with n>1) such as, for example, 1,4-diiodoperfluorobutane (C4F8I2) and 1,6-diiodoperfluorohexane (C6F12I2), can be used as chain-transfer agents in polymerization reactions, as intermediates for the manufacture of other chemicals, including bis-olefins, diacids, polymers or for the manufacture of coatings.
α-Iodoperfluoroalkanes are typically synthesized by means of a telomerization reaction of TFE with a α-iodoperfluoroalkane, preferably CF3I or CF3CF2I. Usually, the most common and industrially convenient way to initiate the reaction is the heating of the starting reactants, the heating temperature being proportional to the length of the desired telomers. The reaction can be carried out in the liquid phase at a temperature of at least 150° C. under pressure. Alternatively, the reaction can be carried out at a temperature as high as 450° C. either at atmospheric pressure, thereby providing only a monoaddition I-telomer in a single step of the reaction, or under pressure, thereby giving rise to a broad distribution of telomers. Methods for the synthesis of I-telomers are disclosed, for instance, in U.S. Pat. No. 3,404,189 (FMC CORP) Jan. 10, 1968, U.S. Pat. No. 5,268,516 U.S. Pat. No. 5,268,516 (ATOCHEM ELF SA) and WO 99/19248 (DU PONT) Apr. 22, 1999.
α,ω-Diiodoperfluoroalkanes are typically synthesized by means of a process comprising the reaction of tetrafluoroethylene (TFE) with iodine in the presence of different initiators, at different temperatures. Also in this case, on an industrial scale it is preferred to initiate the reaction by heating the reactants. TFE promptly reacts with iodine to provide C2F4I2, which is in equilibrium with TFE and iodine; C2F4I2 further reacts with TFE to provide higher-length telomers.
JP S51133206 (ASAHI GLASS CO LTD) Nov. 18, 1976 discloses a process for the preparation of I2-telomers of formula I(C2F4)nI (n=2-4) by thermal decomposition of C2F4I2. The telomerization reaction of TFE with C2F4I2 is disclosed in TORTELLI, et al. Telomerization of tetrafluoroethylene and hexafluoropropene: synthesis of diiodoperfluoroalkanes. J. Fluorine Chem. 1990, vol. 47, p. 199-210.
Example 29 of WO 98/34967 A (THE UNIVERSITY OF NORTH CAROLINA) Aug. 13, 1998 teaches to synthesize a low molecular weight polymer or oligomer [namely CF3(C2F4)nI or F(C2F4)nI] by irradiation of TFE with a UV lamp and heating at 36° C. in the presence of carbon dioxide and of a chain transfer agent such as CF3I or IF.
The main problem in the synthesis of I-telomers and I2-telomers is the handling of TFE, which is flammable and explosive; flammability and explosivity increase with temperature and pressure. Therefore, in the course of the synthesis, it is necessary to properly control temperature and pressure in order not to go beyond critical values. Normally, if temperature is increased, pressure values must be controlled in such a way as not to go beyond 400 kPa at most. This has a negative impact on the productivity of the process. Indeed, under such conditions, less TFE is available in the liquid phase to react with I2, which decreases the reaction speed. In principle, speed could be increased by increasing the TFE pressure or the temperature, but this is not possible due to safety concerns and to the occurrence of undesired side-reactions (i.e. formation of perfluorocyclobutane, TFE polymerization, formation of perfluoroalkanes and I2). Furthermore, the Applicant has observed that reaction mixtures containing TFE and I2, which form C2F4I2 and higher length telomers, are even more flammable and explosive than TFE alone. This is quite surprising, as it was expected that I2 present in such mixtures as a result of dissociation of C2F4I2 and higher length telomers would act as a radical scavenger, thereby increasing the stability of the mixture. The same increased risk of flammability and explosivity is also associated to reaction mixtures containing TFE and CF3CF2I used in the synthesis of α-iodoperfluoroalkanes, since they also give rise to a certain amount of I2, which is known to react with TFE giving rise to C2F4I2.
It is also well known (for example from FERRERO, Fabio, et al. Analysis of the self-heating process of tetrafluoroethylene in a 100-dm3-reactor. Journal of Loss Prevention in the Process Industries. 2012, vol. 25, no. 6, p. 1010-1017.) that a further major concern is represented by the MITD (minimum ignition temperature decomposition) of TFE which decreases by increasing pressure (it decreases from 260° C. at 500 kPa to 230° C. at 1,000 kPa).
EP 702666 A (DU PONT) discloses single-phase, liquid mixtures of TFE and CO2 which are said to be characterised by reduced explosivity. This document focuses on the storage and transport of the mixtures and discloses only in broad terms that TFE/CO2 mixtures can be used directly in TFE polymerization processes or as a diluent/heat sink in chemical reactions involving TFE. However, this document does not suggest any stabilizing effect of CO2 on other systems other than TFE.
There is therefore the need to provide a process for the manufacture of α-iodoperfluoroalkanes and α,ω-diiodoperfluoroalkanes, said process having high productivity and, at the same time, increased safety.
An additional drawback encountered in the manufacture of I-telomers and I 2-telomers, e.g. C4F9I, C4F8I2, C6F13I and C6F12I2, lies in the formation of an undesired side-product, perfluorocyclobutane (cy-C4F8), an inert gas having a boiling point of 6° C. The presence of this side-product in the reactor decreases the reaction speed. In order to overcome this drawback, it is necessary to purge the reactor, so as to remove cy-C4F8, and add further TFE. However, by doing this, not only cy-C4F8 is discharged from the reactor, but also TFE.
JP S53144507 (ASAHI GLASS CO LTD) discloses the preparation of 1,4-diiodo perfluorobutane by thermal decomposition of C2F4I2 in the presence of I2 and an inert gas. The inert gas has the effect of reducing the amount of cy-C4F8 formed in the course of the process. The sole inert gas mentioned in this document is nitrogen.
It would thus be desirable to provide a further, safe process for the manufacture of α-iodoperfluoroalkanes and α,ω-diiodoperfluoroalkanes also allowing to keep to a minimum the amount of cy-C4F8 formed in the reaction. It would also be desirable to provide a process that allows achieving a high productivity of telomers, in particular those having a low molecular weight.